The manufacture of many products requires precise control over temperature and temperature changes. For example, the manufacture of microelectronic devices, such as integrated circuits, flat panel displays, thin film heads, and the like, involves applying a layer of some material, such as a photoresist, onto the surface of a substrate (such as a semiconductor wafer in the case of integrated circuits). Photoresists, in particular, must be baked and then chilled to set or harden selected portions of the photoresist during processing. The baking and chilling steps must be precisely controlled within exacting temperature constraints to ensure that the selected portions of the photoresist properly set with good resolution. Other products and processes involving exacting temperature constraints include medical products and processes including drug preparation, instrument sterilization, and bioengineering; accelerated life testing methodologies; injection molding operations; piezoelectric devices; photographic film processing; material deposition processes such as sputtering and plating processes; micromachine manufacture; ink jet printing; fuel injection; and the like.
Baking typically involves heating a workpiece up to a specific elevated, equilibrium temperature and then maintaining the workpiece at that particular equilibrium temperature for a defined period of time.
Typically, thermal treatment is accomplished by positioning a wafer on a heated platen, also referred to as a bake plate. The heated platen is housed inside an enclosure so that thermal treatment occurs in a protected environment that is isolated from the ambient. It is very important that thermal treatment be as uniform as possible over the full surface area of the wafer. Too much temperature variation over the wafer surface area can adversely impact the performance of the baked photoresist, and hence the quality of the resultant devices.
Thermal uniformity can be quantified in different ways. One suitable approach involves first measuring the surface temperature of the wafer, or of the heated platen that supports the wafer, at several representative points over the full surface of the wafer, or the platen, as the case may be. Thermal uniformity may then be calculated as the difference between the highest and lowest measured temperatures. A typical specification may require that this temperature variation be no more than a fraction of a degree Celsius, for example. As microelectronic devices get smaller and smaller, a trend which is driven by a strong demand for increased miniaturization, temperature uniformity specifications continue to become more stringent. Consequently, there is a continuing demand for more improvement in thermal uniformity.
Conventionally, a number of features have been incorporated into thermal processing stations in order to enhance thermal uniformity. One approach has involved improving the structure and/or features of the heated platen and its related components. Another approach has involved improving the temperature sensor and control systems for monitoring and/or regulating the platen temperature. Still yet another approach has involved enhancing the insulating characteristics of the housing and other components of the thermal processing station to help to thermally isolate the processing chamber from the ambient.
All of these approaches for improving thermal uniformity have been beneficial, yet there continues to be a demand for even better the thermal uniformity performance.
In the past, heating devices have been laminated onto the outside, top surface of a thermal processing station housing, e.g., the lid of the housing, in order to heat the housing and prevent process vapors from condensing on the inside surfaces of the process chamber. For instance, FSI International, Inc. has marketed one thermal processing station with a heated lid in connection with its POLARIS® series tool clusters. However, the heating device used to heat the lid was a flat, heated sheet that was laminated to an outside surface of the lid. Additionally, the lid structure included an alternating series of four relatively flat panels separated by ring structures. This defined a lid structure having, in effect, three stacked plenums constituting at least three air gaps in series between the entire surface area of the heater face at the top of the lid and the process chamber below the lid. Accordingly, this particular lid structure provided little, if any, direct, solid pathways for conducting thermal energy from the heater face uniformly to the process chamber below the heater. The only direct solid pathways for conducting thermal energy from the heater to the process chamber were the exterior side walls of the housing positioned proximal to the outer peripheral edge of the heating device. Moreover, the heating device was used to prevent condensation of the vapors inside the processing chamber and was not used to enhance thermal uniformity of the wafer being processed inside the chamber.